Medical imaging modalities allow the visualization of the organs within a human body. For example, computed tomography (CT) also known as computed axial tomography (CAT): employs X-rays to produce 3D images. There are tens to hundreds of millions of scans done annually worldwide. Although non-invasive, CT is regarded as a moderate to high radiation diagnostic technique.
Another example of medical imaging technology is positron emission tomography (PET) and single photon emission computed tomography (SPECT). PET and SPECT use a short-lived radioactive isotope that undergoes a decay to emit a positron or gamma rays. There are tens to hundreds of millions of diagnostic medical procedures done every year. Both techniques expose the patient to low-level radiation and therefore impose risk to the patient.
A further medical imaging technology is magnetic resonance imaging (MRI). MRI uses a powerful magnetic field to align the nuclear magnetization of protons in water. It provides much greater contrast than does CT. Again, many millions of MRI exams are given annually.
Magnetic resonance imaging (hereinafter referred to as “MRI”) has emerged as a prominent noninvasive diagnostic tool in clinical medicine and biomedical research. Among its many advantages, MRI can produce images with large contrast to visualize the structure and function of the body. It provides detailed images of the body in any plane. MRI generally provides much greater contrast between different soft tissues of the body as compared to other techniques, making it particularly useful in musculoskeletal imaging, cardiovascular and vascular imaging, neurological imaging, oncological imaging and other body parts or functions and diseases. Unlike CT or PET, MRI uses no ionizing radiation, but instead uses a magnetic field to align the nuclear magnetization of atoms (usually hydrogen atoms) in the body. The MRI imaging techniques therefore provide high quality images without exposing the patient to any kind of harmful radiation. The diagnostic power of MRI can be further enhanced with the use of a contrast agent. It is estimated that about 30% of all clinical MRI diagnostic examinations are performed with the intravenous injection of a contrast agent. This constitutes millions of doses of MRI contrast agent administered worldwide annually.
In magnetic resonance imaging (MRI) an image of an organ or tissue is obtained by placing a subject in a strong magnetic field and observing the interactions between the magnetic spins of the protons and radiofrequency electromagnetic radiation. The magnetic spins produce an oscillating magnetic field which induces a small current in the receiver coil, wherein this signal is called the free induction decay (FID). Two parameters, termed proton relaxation times, are of primary importance in the generation of the image. They are called T1 (also called the spin-lattice or longitudinal relaxation time) and T2 (the spin-spin or transverse relaxation time). The time constant for the observed decay of the FID is called the T2* relaxation time, and is always shorter than T2. The T1, T2 and T2* relaxation times depend on the chemical and physical environment of protons in various organs or tissues.
In some situations or tissues, the MRI image produced may lack definition and clarity due to a similarity of the signal from different tissues or different compartments within a tissue. In some cases, the magnitude of these differences is small, limiting the diagnostic effectiveness of MRI imaging. Image contrast is created by differences in the strength of the NMR signal recovered from different locations within the tissue or sample. This depends upon the relative density of excited nuclei (such as water protons), on differences in the relaxation times T1, T2 and T2* of those nuclei. The type of imaging pulse sequence may also affect contrast. The ability to choose different contrast mechanisms gives MRI tremendous flexibility. In some situations, the contrast generated may not adequately show the tissues, anatomy or pathology as desired, and a contrast agent may enhance such contrast. Thus, there exists a need for improving image quality is through the use of contrast agents.
Contrast agents are substances which exert an effect on the nuclear magnetic resonance (NMR) parameters of various chemical species around them. Ordinarily, these effects are strongest on the species closest to the agent, and decrease as the distance from the agent is increased. Thus, the areas closest to the agent will possess NMR parameters which are different from those further away. Proper choice of a contrast agent will, theoretically, result in uptake by only a certain portion of the organ or a certain type of tissue (e.g., diseased tissues), thus providing an enhancement of the contrast, which in turn generates a more accurate image. Contrast agents for MRI that are available may be injected intravenously to enhance the appearance of tumors, blood vessels and/or inflammation for example. Contrast agents may also be directly injected into a joint, for MR images of joints, referred to as arthrograms. Contrast agents may also be taken orally for some imaging techniques. Contrast agents generally work by altering the relaxation parameters, T1, T2 or T2*, such as by shortening these relaxation times.
Since MRI images can be generated from an analysis of the T1, T2 or T2* parameters discussed above, it is desirable to have a contrast agent which affects either or both parameters. Much research has, therefore, centered around two general classes of magnetically active materials: paramagnetic materials (which act primarily to decrease T1) and ferromagnetic materials (which act primarily to decrease T2).
Paramagnetism occurs in materials that contain unpaired electrons which do not interact and are not coupled. Paramagnetic materials are characterized by a weak magnetic susceptibility, where susceptibility is the degree of response to an applied magnetic field. They become weakly magnetic in the presence of a magnetic field, and rapidly lose such activity (i.e., demagnetize) once the external field is removed. It has long been recognized that the addition of paramagnetic solutes to water causes a decrease in the T1 parameter.
Because of such effects on T1 a number of paramagnetic materials have been used as NMR contrast agents. However, a major problem with the use of contrast agents for imaging is that many of the paramagnetic and ferromagnetic materials exert toxic effects on biological systems making them inappropriate for in vivo use. Because of problems inherent with the use of many presently available contrast agents, there exists a need for new agents adaptable for clinical use. In order to be suitable for in vivo diagnostic use, such agents must combine low toxicity with an array of properties including superior contrasting ability, ease of administration, specific biodistribution (permitting a variety of organs to be targeted), and a size sufficiently small to permit free circulation through a subject's vascular system or by blood perfusion (a typical route for delivery of the agent to various organs). Additionally, the agents must be stable in vivo for a sufficient time to permit the clinical study to be accomplished, yet capable of being ultimately metabolized and/or excreted by the subject.
A T1 agent primarily acts to brighten up the tissues where the agent is present due to its ability to enhance the longitudinal relaxation rate of protons from water (1/T1). All the T1 contrast agents currently used in clinical MRI imaging are gadolinium-based paramagnetic complexes with various polyaminopolycarboxylate ligands. 4-8 Gadolinium (Gd) is a rare-earth metal that can form a stable 3+ ion with 7 unpaired electrons (4f7, S=7/2), the highest number of unpaired electrons (or magnetic spins) per metal center obtainable by any metallic element in the periodic table. FIG. 1 shows the structures of several typical Gd-based MRI contrast agents approved for clinical applications so far. The most noticeable feature in all these complexes is the water coordination to the metal center, which provides an important mechanism for enhancing the proton's longitudinal relaxation rate for this water and the surrounding water molecules.
Although gadolinium-enhanced tissues and fluids appear brighter on T1-weighted images, which provides high sensitivity for detection of vascular tissues (e.g. tumors) and permits assessment of brain perfusion (e.g. in stroke), such compounds also have problems and risks. The relaxivity decreases with increasing magnetic field, and thus higher dosages are required to achieve the same contrast with higher magnetic fields. There have been concerns raised regarding the toxicity of gadolinium-based contrast agents and their impact, particularly on people with impaired kidney function. Both the free Gd3+ ions and the polyaminopolycarboxylate ligand molecules used to sequester the metal ions exhibit in vivo toxicity.
Previously, it was assumed that the formation of a chelate between the metal ions and the ligand molecules with high thermodynamic stability and kinetic inertness can prevent the complexes from falling apart, thus reducing the toxicity. Unfortunately, the complex biochemical, pharmacokinetic and metabolic properties of such chelates often render the in vitro working model based on the thermodynamic and kinetic stability considerations inadequate for predicting their in vivo safe delivery. Use of these compounds has been linked to nephrogenic systemic fibrosis (NSF) and nephrogenic fibrosing dermopathy (NFD) for example. The renal toxicity of such agents has also prompted the US FDA to issue a public health advisory regarding the risk of using such agents. Additionally, such compounds are not possible to take orally, requiring intravenous administration, and do not act intracellularly but only extracellularly, thereby limiting their effectiveness.
The second type of contrast agents (i.e. T2 agents) that have been recently approved for clinical use is from the family of iron oxide nanoparticles as shown in FIG. 2. These include superparamagnetic iron oxides (SPIO; 50-500 nm) and ultrasmall superparamagnetic iron oxides (USPIOs; 5-50 nm). In contrast to Gd-based MRI contrast agents, iron oxide nanoparticles can only increase the transverse relaxation rate of protons from water (1/T2), thus producing darkened spots in the tissues where the material is present. From the standpoint of clinical diagnostic imaging, T2 agents produce much less useful information. Thus, the primary application of the T2 agents is for imaging-guided drug delivery and the monitoring of surgical procedures. Such materials have also been used for liver imaging, as normal liver tissue retains the agent, but abnormal areas (e.g. scars, tumors) do not. Other agents such as diamagnetic agents such as barium sulfate have also been studied for potential use in the gastrointestinal tract, but are less frequently used.
It should be noted that both the Gd-based T1 agents and iron oxide-based T2 agents are unstable in the acidic environment of the stomach, which has prevented them from being ever considered for oral delivery. Consequently, these materials can only be intravenously administered. In order to develop any new T1 agent, the water molecules from the surroundings need to be able to exchange with the inner-sphere water molecules, and reside on the metal sites on and off, which can provide a mechanism for water's protons to significantly shorten their T1 relaxation time, thus increasing the proton's magnetic resonance signal intensity (i.e. imaging contrast).
It would be desirable to provide MRI contrast agents which alleviates concerns with known agents and allows high contrast images to be achieved, with low toxicity. It would also be desirable to provide a MRI contrast agent that provides specific biodistribution, cellular imaging and permits free circulation through a patient's vascular system. Further, the qualities of ease of administration, such as by oral delivery methods, and providing stability in vivo for a sufficient time to permit the clinical study to be accomplished, while being ultimately metabolized and/or excreted by the subject, are needed. It would also be advantageous to provide a contrast agent that may allow both T1 and T2 imaging techniques to be performed.
There is also a need for drug delivery materials that allow drugs or other therapeutic agents to be delivered to tissues or portions of the body in an effective manner. There is also a need for agents that allow drugs or other therapeutic agents to be introduced into cells of the body.